1. Field of the Invention
The present invention describes compounds that are inhibitors of IAPs (inhibitors of apoptosis proteins), processes for their preparation, pharmaceutical compositions containing them, and their use in therapy. The compounds of the present invention are useful in the treatment of cancer, autoimmune diseases and other disorders.
2. Description of Related Art
Apoptosis (programmed cell death) plays a central role in the development and homeostasis of all multi-cellular organisms. Apoptosis can be initiated within a cell from an external factor such as a chemokine (an extrinsic pathway) or via an intracellular event such a DNA damage (an intrinsic pathway). Alterations in apoptotic pathways have been implicated in many types of human pathologies, including developmental disorders, cancer, autoimmune diseases, as well as neurodegenerative disorders. One mode of action of chemotherapeutic drugs is cell death via apoptosis.
Apoptosis is conserved across species and executed primarily by activated caspases, a family of cysteine proteases with aspartate specificity in their substrates. These cysteine containing aspartate specific proteases (“caspases”) are produced in cells as catalytically inactive zymogens and are proteolytically processed to become active proteases during apoptosis. Once activated, effector caspases are responsible for proteolytic cleavage of a broad spectrum of cellular targets that ultimately lead to cell death. In normal surviving cells that have not received an apoptotic stimulus, most caspases remain inactive. If caspases are aberrantly activated, their proteolytic activity can be inhibited by a family of evolutionarily conserved proteins called IAPs (inhibitors of apoptosis proteins).
The IAP family of proteins suppresses apoptosis by preventing the activation of procaspases and inhibiting the enzymatic activity of mature caspases. Several distinct mammalian IAPs including XIAP, c-IAP1, c-IAP2, ML-IAP, NAIP (neuronal apoptosis inhibiting protein), Bruce, and survivin, have been identified, and they all exhibit anti-apoptotic activity in cell culture. IAPs were originally discovered in baculovirus by their functional ability to substitute for P35 protein, an anti-apoptotic gene. IAPs have been described in organisms ranging from Drosophila to human, and are known to be overexpressed in many human cancers. Generally speaking, IAPs comprise one to three Baculovirus IAP repeat (BIR) domains, and most of them also possess a carboxyl-terminal RING finger motif. The BIR domain itself is a zinc binding domain of about 70 residues comprising 4 alpha-helices and 3 beta strands, with cysteine and histidine residues that coordinate the zinc ion. It is the BIR domain that is believed to cause the anti-apoptotic effect by inhibiting the caspases and thus inhibiting apoptosis. XIAP is expressed ubiquitously in most adult and fetal tissues. Overexpression of XIAP in tumor cells has been demonstrated to confer protection against a variety of pro-apoptotic stimuli and promotes resistance to chemotherapy. Consistent with this, a strong correlation between XIAP protein levels and survival has been demonstrated for patients with acute myelogenous leukemia. Down-regulation of XIAP expression by antisense oligonucleotides has been shown to sensitize tumor cells to death induced by a wide range of pro-apoptotic agents, both in vitro and in vivo
In normal cells signaled to undergo apoptosis, however, the IAP-mediated inhibitory effect must be removed, a process at least in part performed by a mitochondrial protein named Smac (second mitochondrial activator of caspases). Smac (or, DIABLO), is synthesized as a precursor molecule of 239 amino acids; the N-terminal 55 residues serve as the mitochondria targeting sequence that is removed after import. The mature form of Smac contains 184 amino acids and behaves as an oligomer in solution. Smac and various fragments thereof have been proposed for use as targets for identification of therapeutic agents.
Smac is synthesized in the cytoplasm with an N-terminal mitochondrial targeting sequence that is proteolytically removed during maturation to the mature polypeptide and is then targeted to the inter-membrane space of mitochondria. At the time of apoptosis induction, Smac is released from mitochondria into the cytosol, together with cytochrome c, where it binds to IAPs, and enables caspase activation, therein eliminating the inhibitory effect of IAPs on apoptosis. Whereas cytochrome c induces multimerization of Apaf-1 to activate procaspase-9 and -3, Smac eliminates the inhibitory effect of multiple IAPs. Smac interacts with essentially all IAPs that have been examined to date including XIAP, c-IAP1, c-IAP2, ML-IAP, and survivin. Thus, Smac appears to be a master regulator of apoptosis in mammals.
It has been shown that Smac promotes not only the proteolytic activation of procaspases, but also the enzymatic activity of mature caspase, both of which depend upon its ability to interact physically with IAPs. X-ray crystallography has shown that the first four amino acids (AVPI) of mature Smac bind to a portion of IAPs. This N-terminal sequence is essential for binding IAPs and blocking their anti-apoptotic effects.
Currently, there are drug discovery efforts aimed at identifying compounds that interfere with the role played by IAPs in disease states where a defect in apoptosis is implicated, such as in cancers and autoimmune diseases. Indeed, a number of IAP inhibitors that mimic the interactions of the Smac tetrapeptide are now known and possess pro-apoptotic activity in vitro and in vivo. Some IAP inhibitors demonstrate potent single-agent anti-tumor activity in vitro and in vivo, but unfortunately present an elevated risk of unwanted effects when administered in vivo (e.g., reduced cardiac output and elevated intestinal epithelial apoptosis). Thus, the art continues to look for additional compounds that may function as IAP inhibitors.